1. Field of the Invention
The present invention relates to: a toner suitably used in, for example, electrophotographic methods, electrostatic recording methods and electrostatic printing methods; a developer containing the toner; and an image forming apparatus using the toner.
2. Description of the Related Art
As fixing methods used in dry developing methods, heating heat-roller methods have widely been used by virtue of their energy efficiency.
In order to achieve energy saving by decreasing the fixing temperatures of toners, heat energy applied to toners upon fixation tends to be reduced in recent years.
According to the DSM (Demand-side Management) program of International Energy Agency (IEA) in the 1999 fiscal year, next-generation copiers whose copies per minute (CPM) is 30 or more are required to have such performances that they have to have a waiting time of 10 sec or shorter during which the consumption power is 10 Watt to 30 Watt (which is varied with the copies per minute). Therefore, achieving energy saving has become a quite important issue.
One method proposed for achieving energy saving is making toners melt at low temperatures to thereby decrease their fixing temperature upon use.
In one method for producing such a toner that can be fixed at low temperatures, a binder resin is made to contain a specific non-olefin crystalline polymer or crystalline polyester having a glass transition temperature showing sharp melt property. Also, studies have widely been conducted on methods of achieving both desired low-temperature fixing property and desired heat resistance storage stability by controlling the viscoelasticity and the thermal characteristics of toners.
For example, there has been proposed a toner which contains: a binder resin containing a crystalline resin (especially a crystalline polyester resin) having at least one melting point; and a resin having a water contact angle smaller than that of the crystalline resin (see Japanese Patent Application Laid-Open (JP-A) No. 2002-108018). This proposal describes that improvement in low-temperature fixing property is achieved.
Also, there has been described an image forming method in which image formation is performed using a toner which contains a binder resin containing a crystalline polyester resin as a main ingredient, whereby the low-temperature fixing property of the toner is improved (see JP-A No. 2002-214831).
Furthermore, there has been a proposal referring to the following: a toner which contains as binder resins a crystalline resin and a non-crystalline resin able to at least partially be in a compatible state is decreased in glass transition temperature (Tg) and incorporation of such resins is effective for improving the low-temperature fixing property of the toner; a degree of reduction in glass transition temperature (Tg) of toner is varied with a combination of resins forming a binder resin, and there are combinations with which the low-temperature fixing property can be improved; and hot offset resistance is good and both excellent toner blocking resistance and excellent low-temperature fixing property are achieved (see JP-A No. 2006-276044).
Also, high color-developability has been required for the recent full-color toners for increasing the quality of images. Vivid color development of the fixed toner requires high glossiness of images. In one method of increasing the glossiness of images, spreading property of toner upon fixation is increased by decreasing the viscosity of melted toner, and the surface of an image is more smoothed to increase its glossiness. For example, there has been a proposal of using a crosslinked aliphatic alcohol polyester containing gel and a non-crosslinked aromatic alcohol polyester free of gel as binder resins of a toner in order to achieve both desired fixing property and desired heat resistance property (see JP-A No. 2000-039738).
Also, there has been a proposal of using as binder resins polyester resins having different flow softening temperatures in combination in order to improve fixing property, glossiness and melt-fusion resistance (see JP-A No. 2007-148085).
Also, there has been a proposal of defining a toner containing a polyester resin as a binder resin in terms of the molecular weights of its tetrahydrofuran (THF) soluble matter and its THF insoluble matter in order to achieve both desired low-temperature fixing property and offset resistance (see JP-A No. 2003-280271).
However, the toners of these proposals pursuing to desired low-temperature fixing property may have a problem of sticking between fixed images.
Copiers have been required for high image quality and also for high-speed fixing for improvement in productivity. When fixation is performed at a high speed and many times, fixed paper sheets are discharged before they do not sufficiently release their heat applied thereto upon fixation. That is, a large number of the paper sheets are stacked on top of one another at a stacking portion while carrying a certain degree of heat. In this state, each paper sheet receives intense pressure while keeping heat. Many of toners improved in low-temperature fixing property have a melting property at low temperatures. Thus, paper sheets where such toners have been fixed are stacked on top of one another while the toners are being molten in some degree, causing sticking between paper sheets and image failures such image peeling. Meanwhile, decreasing the viscosity of molten toner to improve its glossiness further raises risk of formation of abnormal images. When cooling is employed as a method for preventing formation of abnormal images, energy for cooling paper sheets is required. As a result, even if energy saving in fixation can be achieved, the total energy saving of the system should be impaired.
In view of the above, in order to prevent formation of abnormal images by focusing on a toner only, JP-A No. 2003-050478 or other documents define a toner such that its DSC curve measured through differential scanning calorimetry (DSC) has an absorption peak in a range of 50° C. to 100° C. in the first heating, and its peak area in the second heating is reduced to ⅓ the endothermic peak area in the first heating. In this toner, the crystalline compound and the binder resin becomes in a compatible state, suppressing sticking between images caused by the crystalline compound.
According to JP-A No. 2007-079329, by using a nucleating agent for a crystalline resin having a freezing point higher than the melting point of a releasing agent, the nucleating agent and the releasing agent are made to be in a non-compatible state, promoting recrystallization of the crystalline resin to improve storageability of a toner and images. This toner, however, is insufficient in achievement of both desired low-temperature fixing property and desired heat resistance storage stability.
Also, JP-A No. 2007-127920 defines the molecular size of a binder resin to make small the difference in molecular size between the binder resin and the releasing agent. With this definition, the releasing agent exudes uniformly and quickly upon fixation and can suppress sticking between discharged paper sheets.
Any of these prior art documents, however, is still insufficient in both sufficient low-temperature fixing property and high heat resistance storage stability.